The passage of a smaller celestial body or its shadow across the disk of a larger celestial body. As observed from Earth, Mercury and Venus are the only planets of the solar system that make transits of the Sun, because they are the only planets with orbits that lie between Earth and the Sun. Mercury makes an average of 13 transits of the Sun each century. Transits of Venus across the Sun are much rarer, with only 7 of them having occurred between 1639 and 2004. In contrast, transits of Jupiter's moons across its disk are common occurrences. Compare occultation.

The passage of a celestial body across the celestial meridian (the great circle on the celestial sphere passing through the celestial poles and an observer's zenith). For any observer, the object is at its highest in the sky at its transit of the observer's meridian. See more at celestial meridian.